retention Flashcards
retention
A term used to describe the RESISTANCE of a denture to LIFTING AWAY from the tissues (VERTICAL DISLODGING FORCES).
- Prevents from being displaced (falling out upper, sticky food dislodging lower)
- Denture lifting away from tissue
“Any portion of the prosthesis that contacts the teeth and helps to prevent removal of the prosthesis.” (Kratochvil )
3 ways to achieve retention
mechanical
muscular forces
physical forces
mechanical means of achieving retention
clasps
- Engage tooth undercuts (below bulbous part of a tooth
- Clasps, guide surfaces, precision attachments (used in conjunction with crowns)
muscular forces means of achieving retention
shape of the denture
- The action of the surrounding musculature on the shape of the non-impression surface (polished surface) of the denture.
- E.g. tongue, lips, cheek
Patient’s muscular control.
- Work in concert to help retain the denture
Through time when wearers are used to the denture. Clasps wear with time but muscular forces will start to work. If denture is made correctly (too bulky in lingual = tongue will dislodge)
physical forces means of achieving retention
coverage of mucosa
Using existing forces of adhesion, cohesion, surface tension and atmospheric pressure on the impression surface of the denture.
Closeness of adaptation, extent (area covered) of the denture base, peripheral seal.
Negative pressure and peripheral seal to keep denture in place via suction
Not so much Co Cr as heavier and more skeletal - less forces
adhesion
surface forces of saliva on denture and mucosa
physical force of retention
cohesion
forces within saliva, viscosity to denture base
physical force of retention
atmospheric pressure
resistance to displacing forces
physical force of retention
2 classifications of retention
direct
indirect
direct retention
resistance to vertical displacement of denture
Clasps, physical forces on oral musculature
indirect retention
resistance to rotational displacement of the denture
- Incorporated in the denture
- help prevent the denture rotating out during function
guide planes
direct retention
- supplementary retention, close to the base and parallel to the path of insertion
good because frictional retention
- No deform over loading, insertion and removal
- Keep the space - no teeth drift
can manufacture if carrying out a restoration e.g. crown made parallel extra orally
how to get direct retention
close to the base, adjacent to the abutment teeth
similar to support
- Interproximal would be great
- But denture material is rigid
Need to utilise undercuts - alter path of insertion
problem with clasps
deform over time and wear
also hard to shape the teeth for them
principal method of providing mechanical retention
clasps
clasp
metal arm that, when in position contacts the tooth preventing removal of the denture base.
clasp placement to be effective as a retainer
placed below the bulbous part of the tooth (undercut).
- Engages into undercut
- Depth is determined by length of clasp
The bulbous portion of the tooth requires identification.
- To accomplish this a SURVEY of the cast is performed.
materials used for clasps
wrought metal and incorporated into the denture base or be included as part of a cast denture base.
2 types of occlusal clasp design
3 surfaces of tooth
bulk next to saddle, retentive component adjacent to the saddle
- good but longer 15mm so maybe more flexible and easily distorted.
OR
terminates at distal,
2 arms one doesn’t engage in undercut and is not retentive - reciprocal arm.
what should clasps be in conjunction with?
a rest (supporting component)
Not necessarily to help in occlusal loading
- But prevent clasp slipping down the tooth and damaging gingival margin
2 ways clasps can approach the surface
occlusally approaching
- terminate in undercut
gingivally approaching
- crossing the gingival margin, from sulcus area up onto the tooth
gingivally approaching clasps
anterior/premolar/canine mostly below lip line, below sulcus
- more aesthetic
- guarantee 15mm in length needed for retention
Terminates determined by undercut
Cross gingival margin
- creates a bridge for infection at gingival crevice - bacteria enter and gather and infection
occlusally approaching clasps
less subtle,
- tend to be used on molars so less obvious
reciprocation
As part of the clasp assembly each clasp unit will have an opposing element to the retentive clasp arm to prevent pressure form the clasp acting on the tooth.
Clasps can cause tooth to move when load applied
- Reciprocal arm helps maintain tooth position
- Opposite side from the clasp
key part of reciprocal arm design
When clasp starts to deform the reciprocal arm needs to be in contact -
Always factor in at design - don’t forget to add when adding clasp
why would you alter the path of insertion and removal?
utilise undercuts to change amount of acrylic needed to not impact on aesthetics
may gain anterior retention with no clasps (desired)
When gain somewhere will lose elsewhere - anterior posterior relationship
- Consider all consequences of altered path
what is the ideal pattern of retention?
3 clasps
1 will help resist the longitudinal plane of the other 2
Angular pattern of retention
- Biggest triangle possible - more stable (think of 3-legged stool)
when would the angular pattern of retention not be possible?
bilateral free end saddle
straight line pattern of retention
anterior undercuts retention is
acrylic not clasps
why should you try and simply/minimise denture design where possible?
anything added to patient mouth is a plaque trap
every component must therefore have a function
how to prevent axial torque on abutment teeth
only pressure when denture is loaded (chewing) should be on rests
- facilitates denture retention
- prevents axial torque
Stress Relieving Clasp System (RPI)
used in free-end saddle designs to prevent stress on the last abutment tooth.
made of
- Mesial Rest
- Proximal guide plate
- Gingivally approaching I-bar clasp
3 components of RPI system
- Mesial Rest
- Proximal guide plate
- Gingivally approaching I-bar clasp
rest component of Stress Relieving Clasp System (RP)
- occlusal mesial rest on the tooth
- rounded on the impression surface
proximal plate component of Stress Relieving Clasp System (RP)
- adjacent to the saddle
- guide surface of 2-3mm
- undercut to permit movement
I-bar clasp component of Stress Relieving Clasp System (RP)
- gingivally approaching
- greatest prominence of the tooth contour
why do maxillary dentures have less need for RPI?
movement restricted by upper hard palate
indirect retention
resistance to rotational displacement of denture
obtained by the extension of a partial denture base to provide the fulcrum of a class II lever. - support leg
how to gain more indirect retention?
Moving support element away from the saddle area =
More retention
Introduce cingulum rests on canines - become supporting legs/pivot points as further away than mesial of last abutment teeth, so more retention
weak indirect retention can occur when
Clasp axis and most anterior supporting element (mesial rest) are close together = dislodge easily (poor indirect retention)
indirect retainers (5)
- Major connectors
- Minor connectors
- Rest
- Saddle
- Denture base
principally supporting components
- rests not clasps
minimum number of clasps for a denture
One clasp on each side of the arch (2 total) should be sufficient depending on the saddle length
when is indirect retention key?
Indirect retention prevents rotation and can be used to provide stability particularly for free-end saddles or very long bounded saddles
when are stress relieving claps system (RPI) predominately used?
mandibular free-end saddle case, but can be utilised in the maxillary arch also.
